A Census of Quiescent Galaxies across 0.5 &lt; <i>z</i> &lt; 8 with JWST/MIRI: Mass-dependent Number Density Evolution of Quiescent Galaxies in the Early Universe

Abstract Recent JWST observations have revealed a large population of quiescent galaxies (QGs) at high redshift ( z ∼ 4–8), challenging current models of early galaxy formation and quenching. Accurate number density estimates are crucial but remain uncertain. We present a systematic study of QGs at 0.5 &lt; z &lt; 8 using a mass-complete sample from the JWST/PRIMER survey with deep NIRCam and MIRI imaging. We demonstrate that MIRI photometry is important for refining the QG sample: it helps to mitigate contamination from dusty star-forming galaxies in the high-mass regime at z ∼ 3–5 and aids in recovering lower-mass QG candidates at z &gt; 5 that are often missed without including MIRI data. We find that the evolution of the QG number density is strongly mass-dependent. The density of massive QGs ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&gt;</mml:mo> <mml:mn>10.6</mml:mn> </mml:math> ) declines rapidly, falling from n ≈ 1.32 × 10 −5 Mpc −3 at z ∼ 3–4 to n ∼ 1 × 10 −6 Mpc −3 at z ∼ 6, and becomes negligible at z &gt; 6. In contrast, low-mass QGs ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>9.5</mml:mn> <mml:mo>&lt;</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>10.6</mml:mn> </mml:math> ) exhibit a constant number density of n ∼ 2 × 10 −6 Mpc −3 across the redshift range z = 4–8. This plateau suggests that these high-redshift, low-mass QGs may be galaxies undergoing temporary quenching episodes, likely subject to rejuvenation upon future gas accretion. Comparisons with leading galaxy formation models reveal significant tensions: most models underestimate the abundance of massive QGs at z &gt; 4 and fail to reproduce the flat density evolution observed for the low-mass population.